The present invention relates to an apparatus and process for the production of a cellular, expanded material based on suitable polymers.
Today, PVC-based rigid foam polymer materials are being widely used as core material in sandwich structures in the naval or aeronautic sector, or as thermal/acoustic insulators in the building sector. In a sandwich structure the core separates two structurally more rigid materials, such as fibre reinforced plastics (FRP), metal or the like. Such sandwich structures have many advantages compared to more traditional single layer structures, such as lower weight, insulation properties etc. Whilst other rigid foam polymer materials, such as foamed polyurethane etc. can be produced using streamlined continuous methods, the production of PVC based rigid foam polymer materials involves moulding of discrete partially expanded bodies (hereafter referred to as embryo bodies) under high pressure in a press. The embryo bodies are subsequently subjected to a chemical-physical treatment to obtain the rigid foam polymer material.
More in detail, the production process of a PVC based rigid foam polymer material initially involves formation of a plastisol paste consisting of a mixture of powders (PVC and other compounds) and liquid substances (in particular isocyanates). The paste is filled in a closed mould cavity and is subjected to a heating and subsequent cooling process under high pressure resulting in a partially expanded embryo body. The embryo body is then further expanded through an additional heat treatment in water and/or a steam oven. The formation of the final rigid foamed material is a result of a hydrolysis reaction of the isocyanate groups present in the material, with the subsequent build-up of a polymer which crosslinks the chemical structure. The moulding process comprises heating the plastisol in a closed mould, whereby a high pressure is created by the thermal expansion of the plastisol and the activation of the blowing agent dissolved therein. The plastisol is kept at elevated temperature for a predetermined period of time in order to allow the plastisol to gelatinize, following which the mould cavity is cooled to a temperature that is low enough to allow removal of the embryo body from the mould without it expanding uncontrollably.
The products obtainable starting from plastisol are characterised by certain ranges of mechanical properties and the foamed products require long gelation times under pressure, long expansion times and long curing times.
As described above, the cellular foamed-polymer products are produced through mixing of powders (PVC, anhydride, chemical blowing agents, pigments) with liquids (e.g. isocyanates and liquid anhydrides and sometimes plasticizers) into a fairly viscous mixture known as a “plastisol”. Said mixture, of high viscosity, after being compounded in a dissolver, is cast into a mould, and the temperature is then increased under pressure, until a temperature of 150° C. to 200° C. is reached, in order to cause the gelation of polyvinyl chloride and the decomposition of the blowing agent to take place. The chemical blowing agents decompose to form gaseous nitrogen that is either dissolved into the newly-formed gel or forms tiny bubbles. This semi-foamed gel is known as an “embryo”. After a predetermined length of time which is sufficient for the embryo to achieve the desired composition the mould and embryo are allowed to cool.
Once the embryo temperature has dropped enough for it to become shape-stable so that it can be released from the mould without damage or uncontrollable expansion, it is transferred to an expansion process unit such as a chamber or tank where it can be foamed, i.e. where it is allowed to expand, to the desired density by being heated in the presence of hot water or steam. The hot water or steam expansion results in a decrease in the viscosity of the embryo as it warms up. Once it has a sufficiently low viscosity, the embryo will expand owing to the pressure of the dissolved nitrogen and additional gas formed through the reaction of the isocyanate content of the gel with the water that diffuses into the gel. The chemical reactions occur both during the initial compression moulding, where the chemical blowing agents decompose and emit nitrogen gas, and during the later expansion when a complex series of water, isocyanate and anhydride reactions occur, giving a final, cured foam with cross linked chemical structure that could be described as a polyamide-polyimid-PVC-polyisocyanurate-polyurea.
It will be apparent from this description that the industrial practice of this batch process is complicated. Furthermore it requires a lot of energy to warm up the metal moulds and press platen in the compression moulding step to form the embryos, and most of this energy is lost when the mould is allowed to cool before releasing the embryo. Furthermore, the moulds have to be moved into and out of the press which, due to their weight and size, is labour intensive and time consuming. Each mould is usually constructed to be able to produce only a single thickness of embryo which limits the flexibility of the system. As the moulds are heated, maintained warm while gelation takes place and then must be cooled before the embryo can be released, the cycle time is high (1.5-2.0 min per mm mould depth). The method only functions satisfactorily when using emulsion polymerised PVC (ePVC) with a high pH value, as the emulsifiers in the ePVC helps catalyse chemical reactions which generate the heat necessary to accelerate the gelling process in the centre of the deep mould used in the process. This reaction also requires silicone-based surfactants and/or quatenary ammonium carboxylate in order to trigger the chemical reactions which generate heat inside the plastisol and accelerate the gelling process. Another reason for the use of ePVC is that it is in the form of small particles which can gel easily without the need of friction forces. The cheaper suspension polymerised PVC (sPVC) cannot be used in this process as it is in the form of large particles which need to be quickly melted using friction in order to generate a homogeneous melt.
A further problem with this process is that it produces undesirably large cells and the formation of the cells is difficult to control as there is no means available to control the pressure inside the mould during the cooling phase.